This study is a prospective, open-label, pharmacokinetic interaction study of the effect of amprenavir on methadone in patients not infected with HIV, but on steady doses of methadone in methadone maintenance programs. Subjects will have a baseline assessment of steady-state 24-hour methadone pharmacokinetics, followed by initiation of amprenavir dosing, 1200 mg q12h. Subjects will have a 24-hour pharmacokinetic assessments ten days later, after sufficient time passes for a new steady state to be established (five half-lives). Subjects will serve as their own controls in a paired analysis of methadone kinetics. Amprenavir kinetics will be compared to subjects from other amprenavir only kinetic studies. Pharmacodynamic assessment (pupil diameter, cognitive function, symptom self report) are measured twice daily during the study.